The use of single-mode optical fibers in communication systems has introduced a number of problems involving polarization sensitivity at the receiver end of the fiber. Present-day single-mode fibers are used to propagate only a single mode at a given frequency. Even if that mode is launched into the fiber in a set linear polarization, at the receiver end, there is no control over the polarization angle, and, indeed, the polarization angle at the output end of the fiber changes seemingly randomly over relatively short times.
Direct optical detectors measure the intensity of the optical signal over an averaging time substantially longer than the period of the optical carrier. Most direct optical detectors are polarization insensitive so that it does not matter which polarization state the mode has assumed. However, the polarization problem arises in wavelength-division multiplexing (WDM) systems in which the fiber carries multiple optical signals at different carrier frequencies. A WDM receiver using direct optical detection must optically filter the multi-frequency WDM signal to pass only the desired channel to the direct optical detector. However, most economical, narrow-band, and tunable optical filters are polarization-sensitive. That is, the degree of transmission will depend on the angle of linear polarization of the light. A WDM receiver using such a polarization-sensitive filter will experience signal fading as the polarization state of the desired channel randomly changes over time. Therefore, there has been a great effort at developing polarization-insensitive and tunable optical filters.
On the other hand, coherent optical detection relies on a local optical oscillator outputting an unmodulated reference signal at the optical carrier frequency. Coherent detection detects the polarization component of the data signal that is in phase with the component of the reference signal having the same polarization. Coherent detection allows the detection of signals of low power levels which direct detection could not distinguish from the noise. However, the lack of control over polarization in the optical fiber requires that the coherent detection be polarization-diversified. Shani et al. have disclosed a polarization-diversified coherent receiver in "Integrated optic front end for polarization diversity reception," Applied Physics Letters, volume 56, 1990, pages 2092-2093. The architecture for such an integrated receiver is illustrated in FIG. 1. A first optical waveguide 10 carries the data signal, and a second optical waveguide 12 carries the local oscillator (LO) signal polarized at 45.degree. to the TE and TM directions. That is, the LO signal contains equal amounts of TE and TM modes. The data and LO signals are combined in a 3 dB splitter 14. Both waveguides 10 and 12 are then carrying equivalent amounts of both signals. The waveguides 10 and 12 are led into respective polarization splitters 16 and 18, in which the TM modes are transferred to third and fourth waveguides 20 and 22 respectively while the TE modes remain on the first and second waveguides 10 and 12. Four direct optical detectors 24 detect the intensities on the four waveguides 10, 12, 20, and 22 and output respective electrical signals I.sub.1, I.sub.2, I.sub.3, and I.sub.4. These four electrical signals can be combined to provide balanced outputs: EQU I.sub.1 -I.sub.2 .varies.(2P.sub.s P.sub.l).sup.1/2 sin .theta.(1)
and EQU I.sub.3 -I.sub.4 .varies.(2P.sub.s P.sub.l).sup.1/2 cos .theta.,(2)
where P.sub.s is the optical power of the data signal and P.sub.l is the optical power of the LO signal, that is, the magnitudes of the vector sum of the respective TE and TM modes, and .theta. is the polarization angle of the data signal relative to the polarization state of the LO signal. The balanced outputs are based on differences in currents and reduce the effect of LO noise. These balanced outputs can then be used to generate the polarization diversified output EQU (I.sub.1 -I.sub.2).sup.2 +(I.sub.3 -I.sub.4).sup.2 .varies.2P.sub.s P.sub.l( 3)
Since P.sub.l is assumed to be constant, the polarization-diversified output provides the time dependence of the data signal P.sub.s even when its polarization state .theta. is varying over time.
The conventional polarization diversified receiver suffers several disadvantages. It requires three splitters 14, 16, and 18, each having approximately the illustrated structure. The splitters occupy more than ten times the surface area occupied by the detectors 24. The waveguides forming the splitters must be bent to separate the optically coupled region from the optically isolated regions. Each bend is large and causes loss in the optical signals.
Weiner et al. have disclosed differences in absorption of TE and TM modes propagating in a waveguide having a central quantum well in "Highly anisotropic optical properties of single quantum well waveguides," Applied Physics Letters, volume 47, 1985, pages 664-667. Although they did not describe any useful devices, they suggested that the effect had potential application to polarization-sensitive devices. Similarly, Choa et al. disclose the polarization sensitivity of an unstrained quantum-well detector in "Optoelectronic properties of InGaAs/InGaAsP multiple-quantum-well waveguide detectors," IEEE Photonics Technology Letters, volume 1, 1989, at pages 376-378. They, however, fail to take advantage of the polarization properties.